What are the potential pitfalls of using cookies for tracking user activity in PHP forums?

Potential pitfalls of using cookies for tracking user activity in PHP forums include privacy concerns, potential security risks if the cookies are not properly secured, and the possibility of users disabling cookies which would hinder the tracking functionality. To address these concerns, it is recommended to use session variables instead of cookies for tracking user activity. Session variables are stored on the server-side and are not accessible to users, providing a more secure and reliable way to track user activity.

// Start a session
session_start();

// Set a session variable to track user activity
$_SESSION['last_activity'] = time();

// Retrieve the session variable to check user activity
$last_activity = $_SESSION['last_activity'];